Growth Control of MoS2 Nanosheets on Carbon ... - ACS Publications

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Growth Control of MoS2 Nanosheets on Carbon Cloth for Maximum Active edges Exposed: An Excellent Hydrogen Evolution 3D Cathode Nan Zhang, Shiyu Gan, Tongshun Wu, Weiguang Ma, Dongxue Han, and Li Niu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 17 May 2015 Downloaded from http://pubs.acs.org on May 18, 2015

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ACS Applied Materials & Interfaces

Growth Control of MoS2 Nanosheets on Carbon Cloth for Maximum Active edges Exposed: An Excellent Hydrogen Evolution 3D Cathode Nan Zhanga,b, Shiyu Gana, Tongshun Wua*, Weiguang Maa,b, Dongxue Hana, Li Niua a

State Key Laboratory of Electroanalytical Chemistry, c/o Engineering Laboratory for Modern Analytical Techniques, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China b

University of the Chinese Academy of Sciences, Beijing 100039, China

Corresponding author: Tongshun Wu; TEL：+86-0431-85262533, E-Mail: [email protected]

KEYWORDS: active-edges control, vertical alignment, three-dimensional, MoS2 nanosheets, hydrogen evolution reaction.

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ABSTRACT To greatly improve the hydrogen evolution reaction (HER) performance, it is the key approach to expose as many active edges of MoS2 as possible. This target is the research hotspot and difficulty of MoS2 which is a promising HER catalyst. In this work, we realized the active-edges control of MoS2 nanosheets on carbon cloth (CC) by growth control during the synthesis procedure. Moreover, MoS2 nanosheets vertically grown on carbon cloth (MoS2⊥CC) was confirmed to be the best morphology with maximum active edges exposed. Multi-factors structure control resulted in abundant active edges exposure and effective electron delivery, thus excellent HER activity. This three-dimensional cathode, MoS2⊥CC, can reach a great current density of 200 mA/cm2 at a small overpotential of 205 mV. The preeminent HER performance can rival the best MoS2-based catalyst ever reported.

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1. INTRODUCTION Hydrogen is clean and sustainable. It can solve the stress situation brought by the pollution to the environment and the increasing energy demands.1-3 Electrochemical splitting of water is a remarkable technique for its abundant source, clean technique and high purity production of hydrogen.4 Although noble Pt-group metals are the most efficient electrocatalyst for hydrogen evolution reaction (HER),5-6 their high cost and resource scarcity hinder the application. MoS2, which is abundant at lower cost, has been considered as one of the most promising catalysts since its hydrogen binding energy is close to that of Pt-group metals by density functional theory calculations.7 However, to achieve ideal catalytic activity of MoS2, two main challenges need to be overcome. Firstly, the HER active sites have been identified to arise from the edges, rather than the basal planes of MoS2 sheet by theoretical calculations and experimental studies.8 To expose more edges, defect rich MoS2,9 oxygen-incorporated MoS2,10 three-dimension (3D) porous MoS2 have been synthesized.11 Secondly, the poor conductivity of MoS2 limits the electron transfer and encumbers the electrocatalytic efficiency to HER. To solve this problem, Lukowski and Voiry have successfully exfoliated 1T-MoS2 as metallic MoS2 with high conductivity.12 13 Many kinds of carbon materials have been employed to synthesize MoS2-based catalysts to conquer these two challenges simultaneously. For example, carbon nanotube (CNT),14 graphene,15-16 carbon fibers,17 amorphous carbon,18 and so on. They can not only improve the electron delivery, but also enfold MoS2 for great active edges exposure against their trend of curling up into inorganic fullerene structures.19 3



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Although MoS2 of different morphologies have been synthesized and supported on kinds of high-conducting materials, the HER activity of MoS2-based material is not satisfactory yet. The exposed active edge sites should be further enlarged and electron transfer efficiency should be further improved. In this way, some substrates, such as mesoporous graphene,20 crumpled graphene balls,21 as well as CNT and graphene together,22 have been designed to construct 3D structures with more active edges exposed. Herein, commercially available carbon cloth (CC) was chosen to be the substrate for MoS2 to form such a 3D cathode. CC has been widely employed in electric field of different applications for their low cost, excellent conductivity, good physical strength and high flexibility.23-25 Meaningfully, the use of CC helped to integrate the electrode into device for applications. In this work, active-edges control of MoS2 nanosheets grown on CC was realized by growth control during the synthesis procedure for the first time. This investigation was pursued from morphology to profound growth mechanism, which builds a comprehensive understanding of the impact of active-edges control on HER performance. In this research, vertical alignment of MoS2 on CC (MoS2⊥CC) was proved to be the optimum morphology with maximum active edges exposure. More importantly, preeminent HER performance has been achieved by MoS2⊥CC. Two main reasons contributed to the outstanding HER performance. Firstly, vertically and densely aligned MoS2 nanosheets on CC can expose almost edges all-around of every MoS2 nanosheet and therefore exposed maximum active sites to the electrolyte. Secondly, the electron transfer efficiency was improved by the involvement of high 4


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conductive CC, and the direct and effective electron delivery from CC to vertically aligned MoS2 nanosheets further. Above all, to the best of our knowledge, it is top highest HER efficient catalyst among MoS2-based electrocatalysts ever reported. 2. EXPERIMENTAL SECTION 2.1. Materials. CC was purchased from Phychemi (Hong Kong) Company Limited. (NH4)2MoS4, Pt/C (20 wt%) and Nafion solution (5 wt%) were bought from sigma Aldrich. N, N-dimethylformamide (DMF) and ethylene glycol (EG), acetone and ethanol were obtained from Beijing Chemicals Corporation, China. Unless otherwise stated, reagents were of analytical grade and used as received. The water used throughout all experiments was doubly distilled water from a Millipore system (>18 MΩ cm). 2.2. Preparation of MoS2⊥CC. CC was cleaned by sonication sequentially in acetone, water and ethanol for 10 min each. The cleaned CC (3 cm×3 cm) was immersed in 30 mL DMF solution containing 29.36 mg (NH4)2MoS4 and stirred for 30 min. Then the solution and CC were transferred into a 50 mL Teflon-lined stainless autoclave and kept at 200 oC for 15 h. After naturally cooled at room temperature, MoS2⊥CC was taken out from the autoclave and rinsed thoroughly with water and ethanol successively several times and then dried in oven at 70 oC for 10 h. The loading for MoS2 on CC was determined to be 0.27 mg/cm2 via a high precision microbalance. In control experiments, different experimental conditions was employed respectively as following: (a) solvent contrast: DMF, water and EG; (b) temperature contrast: 180, 200, 220 and 240 oC; (c) concentration contrast: 45, 29.36, 5



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14.7 and 7.35 mg of (NH4)2MoS4. Pure MoS2 was synthesized by the similar method without adding CC in the autoclave and collected by centrifugation. 2.3. Preparation of physically mixed MoS2 with CC (MoS2-CC) and Pt/C with CC. Pt/C and pure MoS2 were loaded onto CC as working electrodes in contrast experiments. Pt/C ink was prepared by dispersing 1.4 mg Pt/C in 1 mL ethanol containing 35 µL Nafion solution and treated by sonication until homogeneous dispersion was obtained. MoS2 ink was prepared in the same way by pure MoS2 powder. Every working electrode in contrast experiments was fabricated by loading 50 µL catalyst ink evenly on the effective working area of CC (0.5 cm × 0.5 cm) and dried in oven at 70 oC for 10 h. The loading of working electrodes in contrast experiments were all 0.27 mg/cm2, the same with that of MoS2⊥CC. Pt/C is used as the abbreviation of physically mixed Pt/C with CC henceforth. 2.4. Characterization of MoS2⊥CC. Images of scanning electron micrograph (SEM), scanning transmission electron microscopy (STEM) and corresponding energy-dispersive X-ray (EDX) elemental mapping images were obtained on a NOVA NANOSEM 450 FEI scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were carried out on a FEI TECNAI G2 F20 S-TWIN transmission electron microscope operating at 200 kV. The X-ray diffraction (XRD) measurements were recorded in the range of 20−80° (2θ) on a D8 Focus diffractometer (Bruker) with Cu Kα radiation (λ = 0.154 05 nm), operated at 40 kV and 30 mA, and was applied to 6


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investigate the crystallographic structure of the as-fabricated products. X-Ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALAB MKII X-ray photoelectron spectrometer with Al Ka X-ray radiation as the X-ray source for excitation. 2.5. Electrochemical measurements. All electrochemical experiments were performed in the electrolyte of 0.5 M H2SO4 with a three-electrode system. The system consisted of MoS2⊥CC with effective working area of 0.5 cm × 0.5 cm as the working electrode, saturated calomel electrode (SCE) as the reference electrode, and a graphite rod as the counter electrode. When investigating the influence of MoS2⊥CC effective working area on its HER performance, we chose 0.5 cm × 1 cm and 1 cm × 1 cm as the effective working area respectively. Linear sweep voltammetry (LSV) was measured with the scan rate of 5 mV/s. Cyclic voltammetry (CV) was measured with scan rates of 20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 mV/s respectively in potential ranging from 0.34 V to 0.44 V in the investigation of electrochemical double-layer capacitances. During the investigation of stability, CV was performed 500 cycles and then 1500 cycles with a scan rate of 100 mV/s in potential ranging from -0.2 V to 0.2 V. Amperometric i-t curve was obtained at static overpotential of 150 mV. The measurements above mentioned were all conducted by a CHI 760e electrochemical analyzer (CH Instruments, Inc., Shanghai). All data were iR corrected determined by CHI 760e via the resistance test. Electrochemical impedance spectroscopy (EIS) measurements were carried out with Solartron 1255B Frequency Response Analyzer (Solartron Inc., UK) with frequencies ranging from 100 kHz to 7



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0.01 Hz. All the experiments were carried out without activation process and performed at room temperature. All the data are presented relative to a reversible hydrogen electrode (RHE). 3. RESULTS AND DISCUSSION 3.1 Characterization of MoS2⊥CC. MoS2⊥CC was proved to achieve the optimum HER performance through active-edges control investigation (see details in 3.3 chapter). It was successfully fabricated via the solvothermal method using DMF at 200 oC with the precursor concentration of 0.98 mg/mL. After grown with MoS2, the grey color of blank CC (left) turning to black MoS2⊥CC (right) results from the black color of MoS2 as the photographs in Fig. 1a shown. Compared with SEM image of blank CC (Fig. 1b), by the solvothermal synthesis, the successful growth of MoS2 nanosheets on CC with lots of active edges exposed is illustrated by SEM image of MoS2⊥CC (Fig. 1c). In contrast, MoS2 which physically mixed with CC (Fig. 1d) appear to be agglomerating clusters with large amounts of active edges buried. From this point, the solvothermal synthesis of MoS2⊥CC is significant for the achievement of the edge-rich morphology. As the synthesis procedures shown in Fig. 1e, (NH4)2MoS4 is reduced to edge-rich MoS2 nanosheets grown on CC, while agglomerating pure MoS2 are synthesized in the similar way in the absence of CC. In detail, pure MoS2 synthesized without CC are substantiated to fold and agglomerate together. They look like rosemary bud with lots of edges buried inside due to the thermo-dynamically stable status according to their SEM and TEM (Fig. S1a† and Fig. S1b†). In the presence of CC, as exhibited in Fig. 2a, MoS2 nanosheets vertically and 8


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densely grown on CC with large amount of edges exposed. Complementally, TEM image of MoS2 (Fig. 2b) shows an individual sheet obtained from MoS2⊥CC by sonication treatment. It proves that MoS2 nanosheets expand themselves into separated ones when grown on CC. Compared with MoS2 bud, each MoS2 nanosheet grown on CC looks like individual petal and exposes almost all the edge sites around of it, which affirms that we have successfully realized the maximization of exposed edges by CC as a perfect substrate. 0.62 nm interlayer distance shown by the HRTEM image in Fig. 2c corresponds to the (002) plane of MoS2, and 0.23 nm Mo-S distance (Fig. 2d) corresponds to the (100) plane of MoS2.26 The STEM image and corresponding EDX elemental mapping images of C, Mo and S (Fig. 2e) reveal that Mo and S elements are intensively spread each whole carbon fiber. Furthermore, the corresponding layered EDX elemental mapping image (Fig. 2f) straightforwardly and visually substantiates the dense growth of MoS2 nanosheets all around CC. XRD and XPS were measured to characterize the MoS2 grown on CC. XRD patterns of MoS2 scratched from MoS2⊥CC (Fig. 2g) show diffraction characteristic peaks at 15.9, 32.7, 56.8 o corresponding respectively to the (002), (100), (110) planes of the well-defined and hexagonally symmetric structured MoS2. Moreover, the (002) diffraction peak of MoS2 grown on CC is weak and broad as reported,16, 27-28 which indicates more ultrathin MoS2 nanosheets produced with less stacking.18, 26, 29 The chemical state of MoS2 on CC was further confirmed by XPS analysis as shown in Fig. 2h. The binding energy of 228.7 eV, 231.9 eV attribute to Mo 3d 5/2 and Mo 3d 3/2. And the binding energy of 161.6 eV, 162.6 eV corresponds to S 2p 9


3/2

and S 2p

1/2.


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This indicates that Mo4+ and S2- are the dominant states. 3.2. HER performance of MoS2⊥CC. Electrochemical performance of this MoS2⊥CC 3D cathode for HER was demonstrated by LSV measurement. For comparison study, we also tested blank CC, MoS2-CC, and Pt/C. As Fig. 3a shown, Pt/C shows the best HER activity with a negligible onset overpotential, while blank CC hardly exhibits any HER activity in the measurement voltage range. Compared with the onset overpotential of MoS2-CC of 140 mV, MoS2⊥CC exhibits a much lower onset overpotential of 100 mV. Moreover, further negative potential causes a rapid rise of MoS2⊥CC cathodic current density. At 205 mV overpotential, the current density of MoS2⊥CC reaches 200 mA/cm2, which is about 9 times larger than that of MoS2-CC (20 mA/cm2). Furthermore, this result is superior to most MoS2-based electrocatalysts previously reported (Table 1), which certificates that MoS2⊥CC acts as a high-performance 3D cathode for hydrogen generation. In addition, the influence of MoS2⊥CC effective working area on HER activity was further explored. Different currents but the same current density of three electrodes with different effective working areas are obtained as shown in Fig. S2†. This phenomenon suggests that the HER activity is independent of the MoS2⊥CC effective working area. It is also a significant evidence for the uniform growth of MoS2 on CC. To realize low cost of every fabricated cathode, we chose to use small pieces of MoS2⊥CC electrodes with 0.5 cm × 0.5 cm as the effective working area. Even in such a little cathode, large amount of hydrogen bubbles generated and released from it (view video in attachment). 10


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The Tafel plots, which are recorded with the linear regions fitted into the Tafel equation, yielding Tafel slopes of 30 mV per decade for Pt/C, 39 mV per decade for MoS2⊥CC and 49 mV per decade for MoS2-CC respectively as shown in Fig. 3b. The Tafel slope of MoS2⊥CC is also smaller than almost all the MoS2-based electrocatalysts reported recent years as shown in Table 1, which proves the superior HER efficiency of this 3D cathode. Furthermore, Tafel slope value can also imply the mechanism procedure of hydrogen reduction on cathodes.15 In acid solutions, three principal reaction steps as following, equation (1) to (3) are involved with Tafel slopes of 120, 40, 30 mV per decade respectively.18, 30 According to this, Tafel slope of 39 mV per decade obtained by MoS2⊥CC electrode suggests that the Volmer-Heyrovsky mechanism occurred via a fast adsorption reaction (equation (1)) then a electrochemical desorption reaction as the rate determining reaction (equation (2)). Volmer reaction: H3O+ + e + catalyst → catalyst-H + H2O

(1)

Heyrovsky reaction: H3O+ + e + catalyst-H → catalyst + H2 + H2O

(2)

Tafel reaction: catalyst-H + catalyst-H → 2catalyst + H2

(3)

Both polarization curves and Tafel plots show excellent HER efficiency of MoS2⊥CC, which is associated with its morphology and electronic properties. To estimate the effective electrochemically active area of MoS2⊥CC, we employed CV measurement to measure the electrochemical double-layer capacitances (EDLCs), Cdl. Potential range from 0.34 to 0.44 V without faradic current was selected,31-32 which meant the current response in this region was attributed to the charging of the double layer.28 CV curves were obtained at various scan rates in this region (Fig. S3†). The 11



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halves off the positive and negative current density differences at the center of the scanning potential ranges are plotted versus the voltage scan rate in Fig. 3c, in which the slopes are the EDLCs. The Cdl of MoS2⊥CC is 77.89 mF/cm2, more than three times of the Cdl of MoS2-CC (25.34 mF/cm2). The large electrochemically active area confirms the maximized exposed active edge sites due to the dense and vertical growth of MoS2 nanosheets on CC. Apart from the great active edges exposure, efficient electron transfer is also essential for excellent HER performance. As shown in Fig. 3d, the Nyquist plots reveals that the charge transfer resistance (Rct) of MoS2⊥CC (10.29 Ω) is much lower than those of MoS2-CC (93.96 Ω). This very low value corresponds to a fast charge transfer at the interface between electrocatalyst and electrolyte, and results in the excellent electrocatalysis for HER. This ascribed to the involvement of CC as a high conductive substrate to improve the conductivity of the composite. Furthermore, due to the improved effective contact between MoS2 and CC through solvothermal synthesis, electrons can be delivered directly and efficiently from high conductive CC to vertically aligned MoS2 nanosheets. More importantly, as reported, aligning the MoS2 nanosheets perpendicular to conductive substrates optimizes the conductivity of the conductive substrates.28, 33 In addition, the Nyquist plots of MoS2⊥CC electrode at different overpotentials are shown in Fig. 3e. The Rct are 83.37, 27.04, 15.38, 10.29, 5.449 Ω corresponding to 150, 170, 180, 190, 210 mV, respectively. This illustrates more and more rapid electron transfer and electrocatalytic performance for HER with the increased overpotential, which is in agreement with the polarization curve of 12


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MoS2⊥CC. We further inspected the durability of MoS2⊥CC electrode. First, we employed CV measurement for this investigation. Fig. 3f shows the polarization curves of MoS2⊥CC before and after 500 and 1500 CV cycles. Slight decays in current density are observed, suggesting the high stability of MoS2⊥CC. On the other hand, we employed a static 150 mV overpotential and obtained the time-dependent current density curve of MoS2⊥CC (in the inset of Fig. 3f) with slight degradation of the cathodic current degradation during this 40000 s. It reveals the stability under HER conditions of this cathode and the strong binding of MoS2 nanosheets with CC. Furthermore, the cathode is proved to be robust since no change has been observed before (Fig. S4a†) and after 30 min sonication treatment of MoS2⊥CC (Fig. S4b†) according to their SEM images. Overall, these durable tests substantiate the successful fabrication of this stable and robust MoS2⊥CC 3D cathode. 3.3. Active-sites Control. Active-sties control is indispensable for a decent HER performance. As reported, the exposure of active edges can be controlled by adjusting the morphology of MoS2.34-35 Different solvothermal conditions were applied here to realize the active-edges control, which was further convinced by polarization curves. During the synthesis progress, the main effectors to the morphologies of products are solvent,36-39 temperature,40-43 and concentration.43-48 Furthermore, growth mechanisms of different synthesis conditions were analyzed here in order to take a profound insight of the active-edges control in this investigation. Firstly, we investigated the solvent effect of DMF, water, and EG in the 13



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solvothermal procedure, respectively. As Fig. 4a shown, MoS2 nanosheets densely and vertically grown on CC with great active edges exposed as the optimum morphology has been synthesized using DMF at 200 oC with the precursor concentration of 0.98 mg/mL. However, when water is applied as solvent, sparse MoS2 nanosheets slump over CC and attach with each other (Fig. 4b). As Linping Xu suggested, the lower saturated pressure of water which attributes to the lower boiling point contributes to a slower growth and larger size.37 Therefore, MoS2 nanosheets synthesized in water were larger than those synthesized in DMF.39 Consequently, strong interfacial attractive force between substrates and MoS2 influenced the morphology of MoS2 on CC and contributed to the slant of nanosheets. 14, 18, 49 When EG was used as solvent, MoS2 appear to be little balls rather than nanosheets (Fig. 4c). EG has been used in many solvothermal syntheses as both solvent and reducing agent.45, 50-51 (NH4)2MoS4 was reduced to MoS2 by EG in our experiment. As reported, EG was found to bring decrease of the product size and the capping ability of EG helped to reduce exposed areas and form compact structures.36, 52 Accordingly, the small size and compact nanosphere structure of MoS2 attributed to the capping ability of EG53-55 which can coordinate with many cations.36, 50, 52, 56-57 As well known, morphology of MoS2 affects the active edges exposure directly. Therefore, a great diminution of active edges was caused by the oblique and further overlapping of MoS2 nanosheets with water as solvent. On the other hand, lots of active edges hided in tiny nanospheres with EG as solvent. These inefficiently exposed active edges determined the poor HER performance in this system. Cathodes 14


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of MoS2 on CC with water and EG as solvents show lower HER activity than MoS2⊥CC (Fig. 4d). Therefore, DMF is proved to be the appropriate solvent to guarantee effective exposure of active edges. We further investigated the temperature effects at 180 oC, 200 oC, 220 oC and 240 oC, respectively. As shown in Fig. 4e, at 180 oC, nanosheet-structure MoS2 are sparser and less erect on CC than those synthesized through 200 oC (Fig. 4a). This may due to inadequate reduction of MoS2 upon the relative low 180 oC.15, 18, 20, 58 In addition, MoS2 synthesized at 220 oC look like tiny worms spread on the CC with low height and small length (Fig. 4f), and even smaller and lower at 240 oC (Fig.4g). As reported, increasing the solvothermal temperature accelerates the nucleation rate.59 When nucleation is fast and a large number of nuclei are formed, there is limited reagent left for the growth at the same time.60-61 As a consequence, MoS2 obtained at 200 oC were erect and high naonosheets with great active edges exposed. According to Fig. 4h, MoS2⊥CC by 200 oC obtains the best HER efficiency. The concentration of the precursor also affects the morphology and HER efficiency of MoS2 on CC. The concentrations of precursor varied from 0.245, 0.49, 0.98 to 1.5 mg/mL. 0.245 mg/mL results in sparse and slant MoS2 nanosheets overlapping each other as Fig. 4i shown. Compared with 0.98 mg/mL (Fig. 4a), 0.49 mg/mL also achieves vertically aligned MoS2 nanosheets but with lower density (Fig. 4j). As reported, nucleation is mainly controlled by the solution concentration,62 high concentration contributes to the increase of nucleation sites and the product density.46-48 Furthermore, lower nucleation density brings growth forward to the 15



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neighbor sites which means a slant morphology, while higher nucleation density contributes to the vertical growth on substrate due to the limited space.43 Therefore, denser and more erect MoS2 nanosheets with an increasing loading of 0.12, 0.19 and 0.27 mg/cm2 caused more active edges exposure as the concentration increased from 0.245 to 0.98 mg/mL. However, higher concentration of 1.5 mg/mL led to a slightly decreased loading of 0.23 mg/cm2 with a morphology of less erect MoS2 nanosheets in smaller size (Fig. 4k). This may ascribe to the extremely high concentration which made the nucleation too concentrated and too fast to allow enough reagent or time to involve in the growth procedure. It led to smaller size and an uncontrollable growth procedure, thus less erect tiny nanosheets. Correspondingly, Fig. 4l shows that MoS2⊥CC synthesized with the concentration of 0.98 mg/mL achieves highest HER efficiency due to the dense and vertical alignment of MoS2 nanosheets thus maximum active edges exposed. Overall, the vertically aligned morphology obtained by using DMF, 200 oC and concentration of 0.98 mg/mL shows the best HER performance due to the maximum active edges exposure. Furthermore, through the above analysis of growth procedure of different morphologies, a potential growth mechanism of this outstanding cathode, MoS2⊥CC, is presented as following. Due to the adsorption of (NH4)2MoS4 to CC and the strong interaction between MoS2 and carbon materials, (NH4)2MoS4 was reduced to MoS2 nanosheets which tend to nucleate and anchor on the carbon fibers of CC by DMF.16, 18, 28 Due to the proper saturated pressure of DMF, MoS2 nanosheets are more apt to be erective on CC. Additionally, 200 oC as the moderate temperature 16


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for the reduction of MoS2, helped them grew into thin and high MoS2 nanosheets. The proper and relatively high concentration of 0.98 mg/mL led to dense nanosheets which vertically aligned in limited space with great active sites exposed, as scheme 1 illustrated. Overall, through controlling the solvent, temperature and concentration which can influence the nucleation and growth of MoS2 nanosheets, vertical growth of MoS2 on CC with maximum active edges exposed has be successfully synthesized with excellent HER activity. 4. CONCLUSION In conclusion, active-edges control of MoS2 nanosheets grown on high conductive CC was realized by solvent and other conditions control during the solvothermal synthesis. Corresponding HER performances were measured to convince the realization of the active-edges control, and a comprehensive investigation of the growth mechanisms was pursued. This cathode with vertical and concentrated alignment of MoS2 nanosheets on CC was proved to expose maximum active edges and achieve the outstanding HER performance in this investigation. The high density of active edges and excellent electron transfer efficiency were proved to contribute to the excellent HER activity, respectively. MoS2⊥CC was also proved to possess good stability. This 3D flexible robust electrode exhibits eminent HER activity with an onset overpotential of 100 mV and Tafel slope of 39 mV per decade. Most importantly, the current density can reach 200 mA/cm2 at 205 mV overpotential, which can trivial the best MoS2-based electrocatalysts ever reported. ASSOCIATED CONTENT 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Supporting Information. Additional figures giving detailed material characterizations and electrochemical measurements. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGEMENT The authors are most grateful to the NSFC, China (No.21205112, No.21225524 and No.21475122 and No.21127006), the Department of Science and Techniques of Jilin Province (No.20120308, No.201215091 and SYHZ0006) and Chinese Academy of Sciences (YZ201354, YZ201355) for their financial support.

18


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Molybdenum

Sulfide/N-Doped

CNT

Forest

Hybrid

Catalysts

High-performance Hydrogen Evolution Reaction. Nano Lett. 2014, 14, 1228-1233. 27


for


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65. Zheng, X.; Xu, J.; Yan, K.; Wang, H.; Wang, Z.; Yang, S. Space-Confined Growth of MoS2 Nanosheets within Graphite: The Layered Hybrid of MoS2 and Graphene as An Active Catalyst for Hydrogen Evolution Reaction. Chem. Mater. 2014, 26, 2344-2353. 66. Zhou, W.; Zhou, K.; Hou, D.; Liu, X.; Li, G.; Sang, Y.; Liu, H.; Li, L.; Chen, S. Three-dimensional

Hierarchical

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Nanosheets

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Catalytic

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Amorphous

Molybdenum

Sulfide

for

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Figure captions Fig. 1 (a) Photographs of blank CC on the left and MoS2⊥CC on the right. SEM images of (b) blank CC, (c) MoS2 ⊥ CC and (d) MoS2-CC. (e) Schematic solvothermal synthesis of MoS2⊥CC with CC and pure MoS2 without CC. Fig. 2 (a) High-magnification SEM image of MoS2⊥CC, (b) TEM image of MoS2 obtained from MoS2⊥CC, and the corresponding HRTEM images (c) and (d). (e) STEM image and EDX elemental mapping of C, Mo, S for MoS2⊥CC, (f) layered EDX elemental mapping of C, Mo, S for MoS2⊥CC. (g) XRD pattern of MoS2. (h) XPS spectra of Mo 3d and S 2p. Fig. 3 (a) Polarization curves of blank CC, MoS2-CC, MoS2⊥CC and Pt/C, (b) Tafel plots of MoS2-CC, MoS2⊥CC and Pt/C, (c) Linear fitting of the capacitive currents of the catalysts versus scan rates, (d) Nyquist plots of MoS2⊥CC and MoS2-CC at 190 mV overpotential. The inset circuit diagram shows the equivalent circuit model used to fit the experimental data, with empty patterns representing the experiment data and solid line for fitted curve. (e) Nyquist plots of MoS2⊥CC cathode at different overpotentials. (f) Polarization curves for MoS2⊥CC initially, after 500 and 1500 cycles were displayed (inset: Time-dependence of cathodic current density curve for MoS2⊥CC). Fig. 4 SEM images of MoS2 on CC fabricated by (a) DMF, (b) water, (c) EG, (e) 180 o

C, (f) 220 oC, (g) 240 oC, (i) 0.245 mg/mL, (j) 0.49 mg/mL, (k) 1.5 mg/mL and their

corresponding polarization curves (d), (h), (l).

29



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Scheme caption Scheme 1. Schematic illustration of the vertical growth process of MoS2 nanosheets on the surface of carbon cloth.

30


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Table 1 Comparison of HER performance of MoS2⊥CC with other MoS2-based catalyst. ηAt the Onset η Tafel slope Current density corresponding j (mV) (mV dec-1) (j, mA cm-2) Potential (mV)

Catalyst

Reference

Graphene surpported MoS2

~100

41

48

200

15

MoO3-MoS2 core-shell nanowires

150-200

50-60

20

272

63

Mesoporous 3D MoS2

150-200

50

--

--

11

Defect-rich MoS2

120

50

13

200

9

Oxygen-incorporated MoS2

120

55

125.6

300

10

Metallic MoS2

187

43

200

400

12

1T MoS2

100

400

--

--

13

MoS2 Formed on mesoporous Graphene

100

42

30-40

200

20

Network MoS2/CNTs composite

90

44.6

15-20

200

14

MoS2/N-CNT forest

75

40

10

110

64

Graphene supported MoS2 flower

190

95

~30

300

16

CNT-graphene hybrid supported MoS2

140

100

10

255

22

Layer confined MoS2/graphene

~140

41

23

200

65

MoS2/3D-Graphene hierarchical framework

107

86.3

51.6 A/g

400

66

Amorphous MoS2/Nanoporous gold

125

41

5.7

200

67

Li-MoS2/Carbon fiber paper

100

62

200

200

32

S-MoS2 on N-doped carbon nanofibers

30

38

10

120

17

MoS2/N-doped graphene nanosheet aerogels

236

230

10

261

68

MoS2/Amorphous Carbon

80

40

91

200

18

MoSx/Graphene

130

43

17

200

30

MoSx on crumpled graphene balls

130

51.9

220

300

21

MoS2 perpendicular to graphene

--

43

10

172

28

Vertically aligned MoS2 on carbon cloth

100

39

200

205

This work

31



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Fig. 1

32


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Fig. 2

33



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Fig. 3

34


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Fig.4

35



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Scheme 1

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We successfully synthesized MoS2 vertically grown on carbon cloth with maximum active edges exposure and preeminent electrocatalytic efficiency for hydrogen evolution reaction, which can rival the best MoS2-based catalyst ever reported.

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Growth Control of MoS2 Nanosheets on Carbon ... - ACS Publications

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